This invention relates generally to a data recording and reproducing system and, more particularly, is directed to a method and apparatus for recording and reproducing a digitized color video signal on a magnetic tape.
Conventionally, apparatus for recording and reproducing a color video signal have been of the analog, rather than digital, type. The reason for this is that it has previously been believed that the video signal, when digitized, would have an excessively high recording frequency which, in turn, would result in higher magnetic tape consumption. However, due to progress in the field of high density digital recording, it has recently proved feasible to limit the tape consumption to less than or equal to the amount used by analog apparatus. Accordingly, there has been a recent turn towards development of digital video tape recorders (VTR). Digital VTRs have a very high picture quality, which enables multiple generation dubbing with virtually no picture impairment. Further, digital VTRs provide adjustment free circuits and self-diagnosis systems which enable easier maintenance and higher reliability.
With digital VTRs, an analog video signal is converted into digital form by an A/D converter. it should be appreciated that the sampling frequency and the number of quantization levels are the fundamental parameters which determine the quality of the digitized video signal. Further the digitized signal is coded by an error control encoder so that errors may be corrected and concealed on playback, and further, is coded by a channel encoder to achieve high density digital recording. The coded, digitized signal is then recorded on a magnetic tape by means of a recording amplifier.
In one digital VTR, it has been proposed to separate the digitized video signal into at least two separate channels prior to recording it on a magnetic tape. A magnetic head is associated with each channel and all of the magnetic heads are aligned to record the respective channels on a magnetic tape in parallel tracks extending obliquely on the tape. In order to separate the digitized video signal into, for example, two channels, an interface is provided which distributes alternate 8-bit samples of the digitized video signal into the respective channels. Generally, a plurality of such samples in each channel, for example, 96 samples, are formed into a sub-block of data and each sub-block is provided with suitable identifying and address information for identifying the sub-block. A plurality of sub-blocks are then recorded in sequence in each of the two channels. During reproduction, the two magnetic heads supply the information from the two channels on the magnetic tape to another interface which, based on the identification and address information associated with each sub-block, recombines the video signal data from each sub-block to form a continuous digitized video signal.
It is desirable, however, that the digital VTR, like its analog counterpart, have a high speed search mode in which an operator can view the visual information recorded on the tape at a speed which is substantially higher than normal speed. Because of the high tape speed in the search mode, the magnetic heads do not accurately scan the tracks which have been recorded on the tape at the normal speed but rather, scan a plurality of tracks during each scan. Accordingly, an interchanger is provided in the reproducing section for removing the identifying signal from each sub-block of the reproduced signal, and for distributing the signal sub-block by sub-block to the channel to which it belongs. However, by using such interchanger, the reference phase of the color sub-carrier may not be continuously uniform in the high speed search mode. In other words, such reference phase may be inverted between successive sub-blocks. More particularly, in the recording operation, each recorded track preferably includes one field of video information, with each field being comprised of a plurality of lines and each line, in turn, being further divided into, for example, three sub-blocks of video information. During the reproducing operation in the high speed search mode, each head scans a plurality of tracks so as to reproduce signals from different fields. Therefore, if, for example, sub-block signals from an odd frame and an even frame are intermixed, the reference phase of the color sub-carrier may differ at the connection point of such sub-block signals. In other words, it is possible that a phase inversion of the color sub-carrier occurs between successive sub-blocks of information. It is therefore desirable to detect such phase inversion of the color sub-carrier and correct it immediately by, for example, comparison with a reference phase.
Since the phase inversion only occurs in the chrominance portion of the video signal, it is desirable to separate the chrominance portion from the luminance portion of the video signal prior to correcting such phase inversion, without converting the digital signal to an analog signal. It has been found, however, that with the aforementioned distribution of successive digitized samples alternately into two channels during the recording operation, a composite picture having proper color balance cannot be readily obtained during reproduction in the high speed search mode and separation of the chrominance and luminance portions of the video signal in such high speed search mode can also not readily be accomplished.